Present invention relates to self-tapping fasteners, and more particularly to a novel fastener design employing but two thread-forming lobe portions and providing increased radial relief thereby reducing the driving torque required to engage said fastener in a pilot aperture.
There are available numerous forms or types of self-tapping fasteners, such as illustrated in U.S. Pat. Nos. 3,681,963 and 3,218,905. While these fasteners have filled a prior commercial need for self-tapping devices, and have met with a great degree of commercial success, these fasteners are not without their problems or disadvantages, and there exists considerable room for improvement. For example, certain of these prior art fasteners have proved relatively expensive to manufacture; the driving torque required in fully seating the fastener is relatively high, thus necessitating expensive driving tools. Still an additional problem encountered with the prior art design is that of effecting the initial thread-forming engagement with the wall surface of the pilot aperture.
The present invention overcomes a number of these problems, and obviates, to a great extent, other of the problems inherent with the prior art devices. With the present invention, the thread-forming portion on the fastener, i.e., the generally tapered portion adjacent the work entering end, is provided with a specific type of thread form that facilitates initial engagement and materially reduces the driving torque required, without sacrificing quality in the female thread thus formed. In this regard, each thread turn includes but one pair of diametrically opposed lobes which effect cold working of the pilot aperture wall surface. Intermediate these lobes are provided a pair of asymmetrical side portions having a crest height less than the maximum crest height of the lobe portions in a particular thread turn. One of the aforementioned side portions is defined by a controlled gap or void in the thread form, viz., a complete absence of material. The thread turns rearwardly of the thread forming portion are preferably of a conventional design. The radial extent of said controlled gaps decreases in a direction away from said work entering end toward the driving head. As such, the overall cross-sectional area of the tapered, work entering end is reduced, thus facilitating initially engagement of the fastener in the pilot aperture.
The above-discussed thread construction also provides increased radial relief, as compared with the prior art designs, and correspondingly a reduction in the required driving torque for comparable size fasteners. Furthermore, as discussed above and as will be more apparent from the following discussion, this increased radial relief is achieved without sacrificing ease of initial engagement of the fastener in the pilot aperture, or the quality of the female thread formed in the wall of the pilot aperture.
The degree or extent of the gap provided in the thread configuration of the present invention is controlled, such that while the angular extent of the gap increases toward the work entering end of the fastener, the included angle defined by the extremities of the gap is always less than 180.degree.. This feature, as will be explained hereinafter, enables the thread configuration to include a pair of opposed edge portions that are of a crest height less than that of the lobe portion of the particular thread turn. The fact that the height of the edges is controlled in this manner enables the female thread to be formed by cold working of the wall material, rather than by a cutting or metal removal process. More explicitly, maintaining of the discontinuity in the thread turn at less than 180.degree. is achieved by controlling the shape of the blank from which the fastener is rolled. In this regard, a generally circular blank is provided with a slabbed or flattened portion disposed proximate the work entering end and disposed at an angle with respect to the blank axis. The disposition of said plane is controlled, such that a plane containing said flattened portion will not intersect the blank axis short of the work entering end. Thus, any section taken through the blank in the vicinity of the flattened portion would include a flat portion having an angular extent of less than 180.degree..
Thus, when a thread is formed on said blank by a thread-rolling process, there is always sufficient material to produce the above-discussed diametrically opposed lobes, and the thread edges of less height than said lobes, on all thread turns. Such would not be the case, if the flattened portion extended for more than 180.degree.. Assuming a situation wherein the gap extends for more than 180.degree., the thread edges produced would be in the area of the lobes and would be of a relatively sharp nature such that they would engage and bite into the surface of the pilot aperture, acting essentially as cutting edges, and effecting metal removal in the formation of the female thread. By controlling the angular extent of the discontinuity, it is assured that any edges thus produced are of a rounded configuration having a height less than the lobe portion so that only cold working of the aperture wall is employed in the formation of the internal, female thread.